1. Field of the Invention
The present invention relates generally to systems and methods for viewing and imaging the Sun. The present invention relates more specifically to an improved system for the simultaneous imaging of the solar corona and the inner heliosphere from a small, lightweight platform.
2. Description of the Related Art
It is known that activity within the surface layers of the Sun, and in the region of space surrounding the Sun (primarily the corona and near-Sun heliosphere) can have a dramatic affect on both natural and man-made objects on and near the Earth. The monitoring of conditions and activities in the corona and near-Sun heliosphere can provide valuable advance warning of events so as to permit beneficial responses in sensitive systems on Earth and in Earth orbit. One extremely important use of visible light coronal monitoring is for the purpose of space weather forecasting. The existence of around the clock near-real time coronal images has greatly improved an understanding of the physics of space weather and also the ability to predict the affects of features and events that are visible on the disc of the Sun.
Coronal mass ejections (CMES) and their interplanetary manifestations (ICMEs) affect Earth and nearby spacecraft in a number of important ways. These CMEs send accelerating particles into the solar system and affect the shape and topology of the magnetosphere about the Earth. Protons accelerated to high energies by ICMEs are known to be hazardous to spacecraft due to their direct interaction with micro-circuitry, their charging of the spacecraft to high potential, and the space charging of insulators as protons embed themselves in the material. There are actions that can be taken to mitigate the effects of ICMEs on such spacecraft if sufficient warning is provided. Particle acceleration associated with these events is a relatively prompt process with the onset of particle showers occurring minutes to hours after the onset of the CME at the surface the Sun. While some high energy particle events appear to originate in the large scale magnetic X-point associated with a large solar flare, the large majority of these events appear not to originate directly in the solar flares, but begin during the acceleration of the CME through the middle to outer corona of the Sun. This is probably due to the turbulence in the shock wave near the leading edge of the CME.
An operational patrol instrument could provide on the order of one hour's warning of the above described type of proton storm by monitoring the solar corona in real time and detecting the early stages of propagation for fast moving CMEs. The need for this type of measurement motivates both extreme ultraviolet disc imagers and visible coronagraphs which can detect the CME as it lifts off and accelerates through the medium of the solar corona. To be useful for predicting proton storms at the Earth from direct observation of the causing event, coronal measurements must be frequent and prompt. A delay of more than 15-30 minutes can be the deciding factor in determining whether a warning is generated before a storm arrives.
Direct interaction of ICMEs with the Earth's magnetosphere gives rise to geomagnetic storms and associated events. Because of the relatively long propagation time and rich phenomenology of the inner heliosphere, CME behavior at the Sun does not correlate particularly well with ICME behavior or arrival time at Earth. ICMEs can propagate faster or slower than expected by a factor of two, based on initial coronagraph data, and can even engulf one another during their propagation.
Upstream in situ measurements of the solar wind can provide 1-2 hours warning of geomagnetic events (for spacecraft situated near L-1 (Sun-Earth) or in front of it, respectively) by measuring changes in the solar wind pressure and ambient magnetic field before those changes propagate to the Earth itself. However, an understanding of the propagation and evolution of ICMEs in general and of specific events in particular requires monitoring them as they propagate through the inner solar system. Heliospheric monitoring could extend reliable forecasting of major geomagnetic storms from less than two hours (with in situ patrol data) to more than a day (by monitoring the ICME as it propagates through the inner solar system).
The state of the art in near-Sun white light imaging is exemplified by the Solar and Heliospheric Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO) instrument. The LASCO C-3 instrument is an externally occulted coronagraph that collects an image of the outer solar corona several times per hour. Scattered light performance in the LASCO instrument is very good with excellent stability. Instrument cadence, however, is severely limited by available telemetry. LASCO C-3 uses long exposures (19 sec) and as a result is quite susceptible to cosmic ray hits on the detector. These normally take the form of transient spikes but can overwhelm the image during proton storms. This is unfortunate as proton storms occur during the acceleration phase of large CMEs, precisely the time when one is most interested in having good coronal data.
Finally, analysis of LASCO data shows that instrument stability, rather than stray light level, is the most important consideration for operational measurements. CMEs are bright structures, and tracking them is considerably less difficult than, for example, identifying streamers or polar plumes. One benefit of LASCO data is the high stability of the instrument and the platform on which it rests, simplifying analysis and background subtraction.
Wide field heliospheric imaging is a less mature field. Existing and planned missions use two separate strategies, neither of which is wholly suited to low-cost patrol measurement from near Earth. One approach uses three linear detectors that are scanned azimuthally to build up a wide angle image, by the rotation of the host spacecraft. The NASA Solar Terrestrial Relations Observatory (STEREO) Mission Heliospheric Imager (HI) instrument uses a conventional camera lens buried in a baffled corral to achieve extremely low background levels at the expense of field of view. In general this is a good trade-off to make as the STEREO mission is traveling around the Sun and the portion of the corona that is of most interest is near the Earth-Sun line. A further approach involves a hemispherical imager that makes use of annular reflective optics and a shallow corral to image a complete hemisphere of the sky with extremely low scattered light. This latter design, like the corral instrument on STEREO, requires mounting on the side of the spacecraft, defining a particular look direction and requiring two instruments for a heliospheric patrol.
While some efforts in the past have included the use of annular aspheric reflecting optics to reduce stray light in a hemispheric imager, these systems reduce stray light by pointing the imager away from the sun such that the entrance aperture of the instrumentation is shaded by the support structure of the instrument itself. One disadvantage with such designs is that the entire inner heliosphere can not be imaged with a single instrument.
Optical measurements of the solar corona must consistently deal with noise reduction requirements. The dominant source of light in the solar system is the solar disc with a surface brightness eight orders of magnitude higher than the typical mid-coronal brightness, just two degrees from the Sun (as seen from the Earth). The corona itself glows primarily with reflected light from the Sun. There are two components to the visible corona. The Fraunhofer (F) corona (which consists of scattered light from small dust particles near the Sun) is not particularly important for space weather prediction. The continuum (K) corona, on the other hand, consists of Thomson scattered light from electrons near the Sun. The F-corona has between three and thirty times the brightness of the K-corona. Even with a perfect, stray light free instrument, measurements of the K-corona require separating the weak Thompson scattering signal from the much stronger F-corona. Provided that the stray light pattern is fixed, it may be removed by the same background subtraction techniques as the F-corona, so reducing instrumental stray light well below the level of the F-corona may not be a good use of resources for a cost constrained operational mission. Rather, instrument stability, pointing knowledge, and dynamic range become paramount considerations.
It is difficult to overstate the importance of visible-light coronal imaging to space weather forecasting. The benefits and difficulties of coronal imaging are well known in the solar and heliospheric communities and as such provide the motivation and design trade-offs for the development of an improved solar imaging system. The existence of around-the-clock, near-real-time coronal images (from SOHO/LASCO) has greatly improved our understanding of the physics of space weather and also our ability to predict the effects of features and events that are visible on the disk of the Sun. This understanding has highlighted the necessity of such observations on an ongoing basis.